Paper No. 14
Presentation Time: 11:25 AM
ON THE CONSISTENT SEQUENCE OF MINERALOGY AND BEDFORMS IN POST-GLACIAL CAP CARBONATES OF MARINOAN (CA 635 MA) AGE ON DIFFERENT PALEOMARGINS: STEADY-STATE OCEAN STRATIFICATION DURING A POST-SNOWBALL EARTH MARINE TRANSGRESSION?
Cap carbonates atop the Ghaub (Namibia), Puga (Brazil), Stelfox (Canada) and Wilsonbreen (Svalbard) glacials have broadly similar sequences of correlated mineralogy and depth-dependent bedforms. We propose a model of the post-glacial transgression involving steady-state ocean stratification in which an oxic mixed layer is dominated by meltwater and surface heating, and the thermocline reflects prolonged stagnation beneath global ice cover. The stratigraphic sequence begins with peloidal dolomite, deposited wherever the seabed intersected the ocean mixed layer. The sequence (base-to-top) of bedforms within the dolomite is low-angle swaley cross-strata, stromatolite bioherms hosting vertical tubes and gutters, and giant wave ripples. The last represent the deepest part of the mixed layer, where only the longest period waves feel the seabed. The dolomite is overlain by parallel-bedded calcimicrite (or shale) deposited below storm wave base. It contains dolomitic turbidites and local buildups of former aragonite cement (crystal fans). The crystal fans precipitate on the seabed from thermocline water, the calcimicrite settles from overlying surface water, and turbidity flows deliver dolomite from shoreward areas. C and O isotopes display consistent low-temperature equilibrium fractionation between calcimicrite and dolomite, consistent with syngenetic dolomite. The top of the dolomite is enriched in redox-sensitive metals (reactive Fe, Ba, etc), and regionally-extensive primary barite fans were deposited on the seabed in Canada and central Australia. We postulate that barite titrated where euxinic thermocline water, enriched in Ba from submarine weathering of detrital feldspar, mixed into the oxic layer. Upwelling caused sulfate levels to rise >2mM in the mixed layer, inhibiting dolomite formation. Dolomite formed in shallow-water areas, where euxinic thermocline water was absent. Marinoan cap carbonates capture the most complete marine record of any snowball Earth deglaciation.